The work on validation of 13C labeled substrates will be continued to include 13C labeled lactose for use in children with lactase deficiency. Methods for combustion of fecal material to obtain Co2 for isotopic measurements will be developed. Application of 13C labeled aminopyrine to the study of patients with primary biliary cirrhosis under treatment with phenobarbital will be carried out to monitor the effect of the phenobarbital on the liver microsomal mass. Baseline measurements of 13C abundance will be conducted in the resting state, with mild and strenuous exertion to establish normal fluctuation limits for 13C/12C. Day to day variations and the influence of dietary constituents will also be established. BIBLIOGRAPHIC REFERENCES: Sources of Variability in the Use of 13C-Labeled Substrates as "Breath Tests" in Clinical Research and Diagnosis, P.D. Klein and D.A. Schoeller, Z. Anal. Chem. 279:134, 1976. Comparison of Techniques of Collecting Breath CO2 for Isotopic Analysis, D.A. Schoeller and P.D. Kein, Proc. 24th Conf. on Mass Spectrom. and Allied Topics, San Diego, Calif., May 9-14, 1976, pp. 407-408.